Previously, vacuum packaging bags have been proposed in each of which one end is opened and the other end has at least one strip-like bag mouth fused area provided with a non-fused degassing portion(s). When one bag mouth fused area is formed, a sufficient blank area is provided outside the bag mouth fused area. When two or more bag mouth fused areas are formed, they are arranged in such a manner that the degassing portions of the adjacent areas are not in contact with each other.
Contents such as food are put into the bag through the open end, followed by sealing of the open end by means of fusion or clipping. Then, the contents are heated from the inside thereof by microwave heating to complete heat sterilization of the contents for a short period of time. Thus, the contents can be sterilized and vacuum packaged without heat deterioration. Water vapor generated by the microwave heating is evacuated from the bag through the degassing portion(s) formed in the bag mouth fused area(s) and a duct(s) formed between the two or more bag mouth fused areas. The water vapor in the bag and the duct(s) condenses simultaneously with completion of the microwave heating to adhere films forming the duct(s) to each other, thereby closing the duct(s) to prevent air containing various germs from flowing back into the bag. That is, sterilization and vacuum packaging can be easily performed by use of the microwave heating.
According to the above-mentioned method, however, the contents are put into the bag turned upside down. Accordingly, when the contents contain fluid water as in the case of cooked food, it becomes clear that the fluid water enters the duct(s) or leaks to the outside through the open degassing portion(s), resulting in incomplete vacuum packaging.
When the microwave heating is conducted while generating water vapor, the contents are heated from the inside thereof. It has been discovered that the surface of the contents is only heated to about 96.degree. C. to about 97.degree. C. due to vaporization latent heat, although the inside thereof is heated to 104.degree. C. to about 105.degree. C., because the water vapor is generated from only the surface thereof at this time.
When sterilization packaging by the foregoing microwave heating method is attempted, it has been discovered that surfaces of articles to be sterilized are in fact insufficiently sterilized.